Through the calendering of paper, an effort is made to further improve the quality of paper theretofore formed or, in providing a standard level of quality, to achieve a higher running speed or increased bulk of the paper being produced. It is well known that the plasticity or molding tendency of paper may be increased by raising the temperature and/or the moisture content of the paper. A considerable change in plasticity occurs when the temperature of the polymers contained in the paper rises to or beyond the so-called glass transition temperature, at which point the paper may then be more readily molded or formed or finished than it can below that temperature. It is also known that an increase in the moisture content of paper lowers the glass transition temperature. Thus, the paper web is commonly heated in a calender nip by means of a heatable roll, i.e. a so-called thermoroll, and optionally, in addition, by way of a steam treatment before or upstream of the nip. A steam treatment also desirably increases the moisture content of the paper, thereby lowering the glass transition temperature and enhancing the moldability of the paper. However, it is also recognized that at high running speeds the paper does not have an adequate opportunity to be sufficiently heated as it passes through the nip and, moreover, the effects of a steam treatment are typically lost to the environment before the paper reaches the calender.
Thus, the effect of the calendering process on a paper web is highly dependent on the moisture content and the temperature of the fibers contained in and forming the paper at the moment of calendering since the moldability of the fibers markedly, and somewhat disproportionately, increases when their temperature reaches the glass transition temperature and, in addition, the glass transition temperature is directly proportional to the moisture content of the fibers. Above the glass transition temperature it is relatively easy to produce permanent deformations of the fibers whereas, below that temperature, such deformations tend to be reversible. In order to ensure the permanence of the desired calendering effects, the web must accordingly be moistened to lower the glass transition temperature and, in addition, very high calendering temperatures and high pressures must be applied to the web so as to assure that the entire web readily exceeds the glass transition temperature and to provide for uniform deformation of the fibers through the entire cross-section of the web.
The prior art is replete with teachings having the goal of assuring permanence and uniformity of fiber deformation in the production of a paper web. German Patent No. 4,126,233, for example, is directed to a method and apparatus for glazing a paper web. The web is first heated by means of heat radiators so that the web surfaces attain a plasticization temperature, following which the paper web is passed between a pair of rolls which define a nip in which the web is pressed and cooled.
U.S. Pat. No. 5,033,373 discloses a calender including two successively-disposed nips for glazing both (i.e. opposite) sides or surfaces of a paper web. Before entering one of the nips, the paper is cooled by means of a cooling device and, after the web has cooled, that side or face of the web about to contact the hot nip roll is heated by a heating device, preferably by means of a heat radiator or a hot air jet. This heating is intended to make the web surface as hot as possible before it enters the calendering nip.
The art also discloses various methods and apparatus for confining the deformation of the web fibers to only the surface portions of the web. U.S. Pat. No. 4,606,264, for example, provides a method and apparatus for temperature gradient calendering, wherein paper or like material is passed into at least one nip formed by an iron roll and a soft roll. The iron roll is heated to at least that temperature at which the fibers in the web begin to deform; for paper, that temperature is approximately 350.degree. F. As therein disclosed, it is preferred that the web is passed through two successive nips, one for glazing one face of the web and the other for glazing the opposite face.
These prior art methods, however, are neither concerned with nor directed to predeterminately affecting or varying the distribution of moisture within and through a paper web but, rather, merely relate to the distribution of temperature in the web.
It would be notably easier to limit the moldability of the fibers to only the intended surface portions or regions of a paper web if one could assure a transverse moisture distribution in the web characterized by a considerably higher moisture content, at or proximate the surface layer of the paper on that side or face to be calendered, relative to the opposite side or face and to the web interior. Such a moisture distribution would render that side or face of the paper to be glazed substantially more readily moldable than other parts of the web. The typical but largely undesired thinning of the web that results from conventional calendering processes could then be readily minimized.
Moistening of the web with steam may, for example, at least initially be viewed as one possible alternative to solving the aforedescribed problem. Such a procedure, however, often raises other difficulties. Thus, in board machines the temperature of the web before or upstream of a calender is typically approximately 90.degree. C., making it difficult to achieve adequate condensation of steam in the web and to create a clear moisture gradient.
Prior attempts to improve the calendering properties of paper have proposed the addition of microcapsules--which will release the water they contain when subjected to high pressure in a calender nip--to a coating agent that is applied to the paper. Finnish Patent No. 84,509 discloses such a method for moistening a paper web in which water-containing microcapsules are provided in the surface structural layer of the web; the capsules are broken during the calendering process to thereby release water onto the web. The capsules, which comprise a frangible, water-impermeable shell defining a hollow water-containing interior, are added to the coating slip of the paper web. Such procedures have not, however, proven to be entirely satisfactory in practice.
There is accordingly a need in the art for a method of attaining, for use in the calendering process, a predetermined distribution of moisture content in and through a paper web in the thickness direction of the web. The desired internal moisture distribution is such that the web surface to be calendered is considerably moister than the opposite surface of the web and the web interior.
A method of forming a moisture distribution in the web drying art is currently known and marketed under the trademark Condebelt, and is described by way of example in Finnish Patent No. 80,102 and its corresponding U.S. Pat. No. 4,932,139. These references teach a method and apparatus for drying a fibrous web between two substantially parallel metal bands that move in the same direction. The fibrous web is passed or carried, together with a felt, between the opposed moving bands while the band on the web side (i.e. contacting the web) is heated and the band on the felt side (i.e. contacting the felt) is cooled, to thereby dry the web. More particularly, the water present in the web is evaporated by the hot metal band and is then transferred into the felt under the pressure of the resulting steam, simultaneously forcing the water ahead of it. The steam so transferred into the felt condenses by virtue of the cooled band, thus drying the web through the transfer of water from the web into the felt.
No such methods or apparatus for achieving a predetermined or suitable moisture distribution in a web in a calendering process, however, are taught or currently practiced in the art.